Abstract

We studied Ca 2+ -depleted and Ca 2+ -reconstituted spinach photosystem II using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca 2+ ion in the Mn 4O 5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca 2+ -depleted S 1 (S 1') and S 2 (S 2') states, the S 2'Y Z• state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca 2+ -reconstituted S 1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca 2+ -depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca 2+ -containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca 2+ ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca 2+ ion in the OEC is not critical for structural maintenance of the cluster, at least in the S 1 and Smore »2 states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca 2+ removal are discussed, attributing to the Ca 2+ ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to Y Z• (D1-Tyr161).« less

@article{osti_1257848,
title = {Removal of Ca2+ from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study},
author = {Lohmiller, Thomas and Shelby, Megan L. and Long, Xi and Yachandra, Vittal K. and Yano, Junko},
abstractNote = {We studied Ca2+ -depleted and Ca2+ -reconstituted spinach photosystem II using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca2+ ion in the Mn4O5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca2+ -depleted S1 (S1') and S2 (S2') states, the S2'YZ• state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca2+ -reconstituted S1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca2+ -depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca2+ -containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca2+ ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca2+ ion in the OEC is not critical for structural maintenance of the cluster, at least in the S1 and S2 states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca2+ removal are discussed, attributing to the Ca2+ ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to YZ• (D1-Tyr161).},
doi = {10.1021/acs.jpcb.5b03559},
journal = {Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry},
number = 43,
volume = 119,
place = {United States},
year = 2015,
month = 5
}

Ca 2+ -depleted and Ca 2+ -reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca 2+ ion in the Mn 4 O 5 Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca 2+ -depleted S 1 (S 1') and S 2 (S 2') states, the S 2'Y Z • state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca 2+ -reconstituted S 1more » state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca 2+ -depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca 2+ -containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca 2+ ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca 2+ ion in the OEC is not critical for structural maintenance of the cluster, at least in the S 1 and S 2 states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca 2+ removal are discussed, attributing to the Ca 2+ ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to Y Z • (D1-Tyr161).« less

High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopymore » (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3dz2-4pz mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3dxz,yz transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.« less

High-valent Mn=O species have been suggested to have acatalytically important role in the water splitting reaction which occursin the Photosystem II membrane protein. In this study, five- andsix-coordinate mononuclear Mn(V) compounds were investigated by polarizedX-ray absorption spectroscopy in order to understand the electronicstructure and spectroscopic characteristics of high-valent Mn species.Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were alignedalong selected molecular vectors with respect to the X-ray polarizationvector using X-ray diffraction. The local electronic structure of themetal site was then studied by measuring the polarization dependence ofX-ray Absorption Near Edge Spectroscopy (XANES) pre-edge spectra (1s to3d transition) and comparing withmore » the results of density functionaltheory (DFT) calculations. The Mn(V)-nitrido compound, in which themanganese is coordinated in a tetragonally distorted octahedralenvironment, showed a single dominant pre-edge peak along the Mn?N axisthat can be assigned to a strong 3dz2 - 4pz mixing mechanism. In thesquare pyramidal Mn(V)-oxo system, on the other hand, an additional peakwas observed at 1eV below the main pre-edge peak. This component, whichdoes not show noticeable dichroism, was interpreted as a 1s to 3dxz, yztransition with 4px,y mixing, due to the displacement of the Mn atom outof the equatorial plane. The XANES results have been correlated to DFTcalculations, and the spectra have been simulated using a TD (timedependent)-DFT approach. The relevance of these results to understandingthe mechanism of the photosynthetic water oxidation isdiscussed.« less

The structure of the Mn complex of photosystem II (PSII) was studied by X-ray absorption spectroscopy. Oxygen-evolving spinach PSII membranes containing 4-5 Mn/PSII were treated with 0.8 M CaCl/sub 2/ to extract the 33-, 24-, and 16-kilodalton (kDa) extrinsic membrane proteins. Mn was not released by this treatment, but subsequent incubation at low Cl/sup -/ concentration generated preparations containing 2 Mn/PSII. The Mn X-ray absorption K-edge spectrum of the CaCl/sub 2/-washed preparation containing 4 Mn/PSII is very similar to the spectrum of native PSII, indicating that the oxidation states and ligand symmetry of the Mn complex in these preparations aremore » not significantly different. The Mn extended X-ray absorption find structure (EXAFS) of CaCl/sub 2/-washed PSII fits to a Mn neighbor at approx. 2.75 A and two shells of N or O at approx. 1.78 and approx. 1.92 A. The results demonstrate that the structure of the Mn complex is largely unaffected by removal of 33-, 24-, and 16-kDa extrinsic proteins, and thus these proteins do not provide ligands to Mn. The Mn K-edge spectrum of the CaCl/sub 2/-washed sample containing 2 Mn/PSII has a dramatically altered shape, and the edge infection point is shifted to lower energy. The position of the edge is consistent with a Mn oxidation state of +3. The Mn EXAFS of this preparation is also quite different and cannot be simulated by using the parameters for the native Mn complex. Thus the structure of the Mn complex is disrupted upon depletion of half of the Mn.« less

X-ray absorption spectroscopy (XAS) has been used to characterize the structural consequences of Ca{sup 2+} replacement in the reaction center complex of the photosynthetic oxygen-evolving complex (OEC). EPR and activity measurements demonstrate that, in the absence of the 17 and 23 kDa extrinsic polypeptides, it is not necessary to use either low pH or Ca chelators to effect complete replacement of the active site Ca{sup 2+} by Sr{sup 2+}, Dy{sup 3+}, or La{sup 3+}. The extended X-ray absorption fine structure (EXAFS) spectra for the OEC show evidence for a Mn...Mn interaction at ca. 3.3 A that could arise either frommore » Mn...Mn scattering within the Mn cluster or Mn...Ca scattering between the Mn cluster and the inorganic Ca{sup 2+} cofactor. There is no significant change in either the amplitude or the phase of this feature when Ca{sup 2+} is replaced by Sr{sup 2+} or Dy{sup 3+}, thus demonstrating that there is no EXAFS-detectable Mn...Ca contribution at ca. 3.3 A in these samples. The only significant consequence of Ca{sup 2+} replacement is a small change in the ca. 2.7 A Mn...Mn distance. The average Mn...Mn distance decreases 0.014 A when Ca{sup 2+} is replaced by Sr{sup 2+} and increases 0.012 A when Ca{sup 2+} is replaced by Dy{sup 3+}. 75 refs., 10 figs., 5 tabs.« less